Abstract

The main diagenetic events of polyphase rift valley successions are controlled by provenance geology, tectonic evolution and subsidence, weathering related to different episodes of fresh water flushing, and topographic and climatic effects. Based on the Tanzanian examples as case studies and comparisons with published studies on rift valley sedimentation and diagenesis, the main diagenetic phases of Tanzanian rift valley successions can be presented:

  1. Early diagenetic precipitation of evaporites, hematite and calcrete along with feldspar dissolution, and in some cases formation of pore-filling clay minerals, zeolites and feldspar, may take place.

  2. Middle diagenetic formation of albite, zeolites, chlorite, recrystallization of calcite and some quartz precipitation have been reported from several rift valley settings. Continuous subsidence results in enhanced recrystallization of calcite, with a trend towards larger crystal size and more poikilotopic configurations, and new generations of quartz and clay minerals such as e.g. illite. The precipitation of dolomite, ankerite and siderite have commonly been observed during this stage.

  3. Late diagenetic processes is related to uplift and renewed tectonic activity. Under such conditions new generations of hematite, calcite and clay minerals were formed, partly as a result of feldspar dissolution caused by meteorically-induced fresh water flushing.

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